Prophage activity shapes the thermal ecology and evolution of a marine bacterial host

Predicting the response of populations to thermal change is challenging due to temperature-dependent interspecific interactions, such as those between bacteria and their integrated viruses (prophages). Temperature is known to modulate prophage activity, raising the question of whether and how prophages alter host responses to temperature. In this work, we explored how prophage infection shapes host thermal ecology and evolution in a novel marine bacterial isolate carrying a spontaneously inducing prophage. We first characterized the effect of temperature on bacteria and phage population dynamics, finding that high temperatures led to bacterial population crashes and reduced yield, with abundant phage production across all temperatures. To then determine the causal impacts of this phage, we infected a conspecific susceptible strain and found that performance in the novel lysogen was reduced across all temperatures and indistinguishable from the naturally infected strain. Further, we found that evolutionary rescue mutants of the naturally infected strain with an expanded upper thermal limit gained high temperature adaptation through a reduction in prophage activity. Overall, we found that intraspecific variation in thermal ecology was explained by prophage infection, and that adaptation to nonpermissive temperature occurred through modifications to prophage dynamics. This work demonstrates a critical link between viral mobile genetic elements and bacterial thermal responses, and emphasizes the importance of ecological interactions in thermal ecology and evolution.